The present invention relates to a device, a method and a lighting system for detection of defects in and/or on pellets and for sorting of pellets, preferably made of plastics, which differ from a predetermined external appearance.
It is previously well known how pellets can be sorted on the basis of a predetermined external appearance. The term pellets refers to ball-shaped or granular objects. Occasionally, pellets may also be called granules. The pellets are preferably made of plastics, but may also be constituted by other materials, such as glass, grains, potatoes, nuts etc. The difficulty as regards the sorting of pellets is, among other things, the actual detection of a defect which may occur in or on a pellet due to the shape or the external appearance (e.g. a certain colour) of the pellet. One reason for why it may be difficult to detect a defect in or on a pellet is that the detection should be evenly carried out over the entire pellet (inside as well as outside), in order to obtain a great accuracy in the detection and to avoid disturbances such as xe2x80x9cshadowsxe2x80x9d and/or reflections (i.e. sections which may be construed as a defect but actually are is just a shadow and/or a reflection). This type of measurement is often very costly and complex. The defects which may occur are, for example, air bubbles, impurities, a differing symmetry, a differing colour etc.
One method of sorting pellets is to first feed the pellets on a conveying belt through feeding means and to convey the pellets on a conveying belt. The feeding means can be constitued by a container for pellets and a mouthpiece in connection with the container which provides a feeding of the pellets. The conveying belt may, for example, be constituted by an endless driving belt. Thereafter, defect pellets which are conveyed on the conveying belt can be detected by means of separation means, i.e. some sort of a detection device. Defect pellets are sorted out by means of sorting means, wherein the sorting means for example can be constituted by a release device for pellets which cooperates with the separation means.
One example of how defects in objects can be detected is described in the U.S. Pat. No. 5,201,576, wherein the detection, is carried out in a spherical chamber which is covered with a highly reflective colour in order to obtain a system which is free from shadows. Furthermore, the chamber is provided with a circularly tubular lamp, two video cameras and a transparent, cylindrical tube having two open ends. The objects are conveyed through the tube, illuminated by the lamp and examined by the cameras. One problem which may occur in connection with this solution is that it may be difficult to adjust the cameras without affecting the light distribution inside the chamber. This is due to the fact that the intensity from the lamp, which is described in the U.S, Pat. No. 5,201,576, will vary inside the chamber, which is due to the fact that the intensity is higher close to the lamp than at a certain distance from the lamp. Another problem may be that the tube affects the light refraction in the form of reflections, i.e. that a mirror image of the lens may appear. Also, this resolution is very complex and costly, while it at the same time only can detect one object at the time.
Another example of how pellets can be sorted is described in EP-A2-0 705 650, wherein the pellets detection is carried out with regard to the colour of the pellets. It seems, however, that the detection only can determine whether the pellet has a differing colour and that the detection thus is unable to determine whether a defect is present inside or outside the pellet, in the form of, for example, air bubbles, impurities, a differing symmetry etc. Also, the detection in EP-A2-0 705 650 seems to have a less even light distribution, which may give rise to disturbances in the detection, such as shadows and/or reflections.
A device and method for sorting of pellets so that pellets containing defects are detected and sorted out. The device includes a light detector arranged over a transparent pellet transport track. A light source is arranged on the opposite side of the track. The detector provides a measurement of a received light intensity, and if the measured intensity is lower than a predetermined threshold value, it can be assumed that a defect is present. The pellet containing the defect is then sorted out by actuating a sorting device. In order to obtain a high precision detection, light is distributed evenly from all directions, including ambient light. The light source fine tunes the balance of the light source and incoming light from other directions. A highly reflective chamber and/or tilted optical axis further provide the desired even distribution of light.
Detailed preferred embodiments of the method and the device according to the invention will be apparent from the appended dependent claims.